1. Field of the Invention
The present invention relates to an adapter correcting for glass thickness and to a field lens to which the adapter correcting for glass thickness is attached.
2. Description of Related Art
Conventionally, a field lens accommodating a plurality of lenses within a lens barrel is known which is capable of correcting, through design of the field lens, aberrations due to thickness of a glass plate when an objective is observed through the glass plate. However, such a field lens may be unable to achieve adequate optical performance in a case where the thickness of the glass plate between the objective and the field lens varies.
In response, a configuration is known in which an adapter correcting for glass thickness is provided to a field lens correcting for glass thickness as a separate component (see, e.g., Japanese Utility Model Publication No. S61-016526). The adapter correcting for glass thickness described in Japanese Utility Model Publication No. S61-016526 includes an attachment frame and a plane parallel plate. The attachment frame is capable of attachment and detachment with respect to a forefront end of a lens barrel of the field lens. The plane parallel plate is held by the attachment frame. The adapter correcting for glass thickness is attached to the field lens according to the thickness of the glass plate interposed between the field lens and the objective. For example when a glass plate having a thickness of 2 mm is interposed between the objective and a field lens designed assuming interposition of a glass plate having a thickness of 5 mm, an adapter correcting for glass thickness is mounted to the lens barrel of the field lens, the adapter including a plane parallel plate having a thickness of 3 mm. Accordingly, the objective can be observed with the field lens with an adequate optical performance even when the thickness of the glass plate is 2 mm.
Alternatively, a field lens is known which corrects for aberrations corresponding to thickness of a glass plate by displacing a lens within the field lens along an optical axis (see, e.g., Japanese Patent No. 2839263). The field lens of Japanese Patent No. 2839263 corrects aberrations due to the thickness of glass in the glass plate interposed between the objective and the field lens by displacing one of three lens groups.
By attaching the adapter correcting for glass thickness to the field lens, the configuration described in Japanese Utility Model Publication No. S61-016526 specifies the thickness of a glass plate that can be accommodated. For example, when the adapter correcting for glass thickness includes the plane parallel plate having a thickness of 2 mm and the adapter is attached to the field lens designed assuming interposition of the glass plate having a thickness of 5 mm, only a glass plate having a thickness of 3 mm can be accommodated. However, cases where the thickness of the glass plate is 1 mm, 2 mm, or 4 mm, or a case where no glass plate is included, cannot be accommodated. Accordingly, in order to accommodate glass plates having these thicknesses, separate adapters correcting for glass thickness must be prepared having plane parallel plates of 4 mm, 3 mm, 1 mm, and 5 mm, thus increasing costs.
By attaching another, different adapter correcting for glass thickness to the attachment frame of the adapter correcting for glass thickness according to Japanese Utility Model Publication No. S61-016526, different glass plates can be accommodated. For example, when adapters correcting for glass thickness having plane parallel plates of 1 mm, 2 mm, and 3 mm are respectively prepared, each adapter is employed individually. Thereby, glass thicknesses of 4 mm, 3 mm, and 2 mm can be accommodated. In addition, by combining these adapters, glass thicknesses of 5 mm, 4 mm, and 3 mm can be accommodated. However, the attachment frame of the adapter correcting for glass thickness according to Japanese Utility Model Publication No. S61-016526 includes a mount for fixating the attachment frame to the lens barrel of the field lens. Thus, when several attachment frames are used, the size of the adapter increases by the size of the mounts. In such a case, a distance between the objective and a surface of the adapter closest to the objective is reduced. Accordingly, the distance to which the field lens is able to advance and retreat with respect to the objective (operating distance) is also reduced. Adequate focus adjustment may therefore be impossible and functionality of the field lens may be reduced.
In addition, in a configuration where the lens position within the field lens is displaced, as in Japanese Patent No. 2839263, the configuration displacing the lens may become complicated and increase costs.